Big Block Functional Differences :: 1965 - 1976


Big Block Functional Differences

For those seeking to build a really heavy-duty engine, rumor has it that the blocks with an "F5" or "F6" cast on the rear face, under the flexplate area, are higher in nickel, and stronger. These are commonly 1967 to 1970 or so 425 or 455 blocks.

Year CID Nickel Content Code
'68 - '70 350 F4
'68 455 F4
'68 455 F2
'70 455 F1
350 Diesel F6

You can use either a 455 or a 425 block to make an engine of either displacement; the bores are the same. What's different is the crank [stroke], rods (length), and pistons (compression height). So get the crank, rods and pistons! Also, different combinations of lifter diameter and that cam bank angle were used. The 425 "D" blocks come in two configurations: Toronado units have larger lifters, set on a more modern bank angle, whereas more mundane [non-Toro] 425 blocks have the common smaller lifters set on the early cam bank angle. The Toronado block is generally preferred because of the more common lifter bank angle, and the other perks that a Toronado engine brings: big valves, etc.

All engines 1968 and up, (all 455's, 400's, 403's, 350's, 307's, 260's), use the same lifter bank angle of 39°. All 455's and most small blocks [all but late 307's?] use 0.842" diameter lifters. Most pre 1968 engines use the same lifter, but set on a 45° bank angle. Some pre 1968 engines used larger 0.921" lifters, notably 442 and Toronado engines.

KEY: The cam must be ground to match the block's cam bank angle. That cam will then work in any Olds engine with the same bank angle, but will NOT work in blocks with the other bank angle. Either lifter diameter can be used with either cam, just get lifters to match the block. Which brings up two important differences: the 0.921" lifters are rather more costly, at about $100 a set vs. $30 a set for the more common units, and the larger lifters require shorter pushrods. Also, some cam manufacturers no longer offer cams in the 45° form. Contact Engle for custom cams, as they grind all Mondello's cams.

[ Thanks to Chris Witt for this information ]